1. Field of the Invention
The present invention relates to optical components applicable to optical communication systems and, more particularly, to an optical fiber grating element provided with a long-period grating in a multi-mode optical fiber, a production method thereof, and an optical fiber filter including the same.
2. Related Background Art
An optical fiber grating element provided with a long-period grating (LPG: Long-Period Grating) in a core is an optical component that couples core-mode light of a predetermined wavelength to cladding-mode light by the long-period grating to attenuate the light of the predetermined wavelength. In other words,the optical fiber grating element is the optical component that selectively transfers the power of the core-mode light of the predetermined wavelength to the cladding-mode light (for example, see A. M. Vengsarkar, et al., J. of Lightwave Tech., Vol. 14 (1996) pp58-64). Here the core-mode light is light propagating as being confined in the core region of the optical fiber. On the other hand, the cladding-mode light is light radiating from the core into the cladding of optical fiber. Such optical fiber grating elements are utilized as optical fiber filters or the like for selectively cutting off the core-mode light of the predetermined wavelength (loss wavelength) out of the core-mode light of a wavelength band in use having propagated in the optical fiber, in the fields of optical communications and the like.
Cladding modes mean high-order modes except for the fundamental mode when consideration is given to the entire region of the optical fiber specified by the cladding surface being the outermost layer or by the interface between the cladding and a coating layer covering the cladding. For example, in single-mode optical fibers, there exist the fundamental mode with consideration to propagation only in the core and the high-order modes with consideration to propagation in the entire region A of the optical fiber, as illustrated in FIG. 1A. In the case of the single-mode optical fibers, therefore, change in the refractive indices of the surroundings around the cladding (the refractive indices of the air layer and the coating layer) will also cause a shift of the wavelength at which the coupling occurs from the core-mode light to the cladding-mode light, i.e., a shift of the loss wavelength in the long-period grating. There will also occur variations in attenuation factor of the core-mode light of the loss wavelength. Particularly, it is known that when the peripheral surface of the multi-mode optical fiber is covered with a resin having the refractive index close to glass, the high-order modes (cladding modes) disappear as illustrated in FIG. 1B (for example, see B. H. Lee, et al., OECC""98, 14P-50 and B. H. Lee, et al., Electronics Letters, Vol. 34 (1998) pp1129-1130) . For that reason, it was infeasible to cover the optical fiber grating element provided with the long-period grating in the multi-mode optical fiber, with a coating for the purpose of protection for the element.
In order to overcome this problem, the optical fiber grating element described in above B. H. Lee et al. comprises the long-period grating in the single-mode optical fiber having the index profile of dual shape core (DSC) structure the base of which is silica. Here the single-mode optical fiber of DSC structure is composed of a first core region of a refractive index n1, a second core region of a refractive index n2, and a cladding region of a refractive index n3 in the order named from the center of the optical axis (where n1 greater than n2 greater than n3). The first and second core regions of the single-mode optical fiber both are doped with GeO2 and these first and second core regions are exposed to ultraviolet light spatially intensity-modulated, thereby obtaining the optical fiber grating element in which an index-modulated area or a grating is formed across these two regions. The optical fiber grating element, which is obtained by providing the long-period grating in the single-mode optical fiber as described, couples the core-mode light of the predetermined wavelength propagating in the first core region to the high-order mode (cladding-mode) light, so as to cut off the core-mode light of the predetermined wavelength.
This long-period grating is a grating that induces coupling (mode coupling) between the core mode propagating in the optical fiber and the cladding mode, as elucidated in U.S. Pat. No.5,703,978, and that is definitely discriminated from the short-period gratings that reflect the light centered about the predetermined wavelength. In the long-period grating, in order to achieve strong power conversion from the core mode to the cladding mode, the grating period (pitch) xcex9 is set so that the difference between propagation constants of the core-mode light of the predetermined wavelength (loss wavelength) and the cladding-mode light becomes 2xcfx80/xcex9. Since the long-period grating acts to couple the core mode to the cladding mode in this way, the core-mode light attenuates in a narrow band centered around the predetermined wavelength (loss wavelength).
Inventors studied the above conventional techniques and found the following issues. First, it is extremely difficult to design and fabricate the optical fiber grating element with desired cutoff characteristics (loss wavelength and loss amount) by the technology described in the above documents of B. H. Lee et al. The reason is that changes of index in each of the first and second core regions based on the exposure to ultraviolet light are greatly affected by the conditions of the intensity of the radiant ultraviolet light, the exposure time, and so on, or by the conditions of a pretreatment of the optical fiber to be exposed to the ultraviolet light.
In addition, since it is difficult to predict the changes of index in each of the first and second core regions, it is extremely difficult to accurately control the fabrication of the optical fiber grating element so that both the loss wavelength (cutoff wavelength) and loss amount (cutoff amount) fall at designed values or within a designed range.
The present invention has been accomplished in order to solve the problems described above and an object of the invention is to provide an optical fiber grating element of structure permitting more precise design and fabrication, a production method thereof, and an optical fiber filter including the same.
An optical fiber grating element according to the present invention comprises a multi-mode optical fiber having a cutoff wavelength regarding to LP02-mode light on the longer wavelength side than a wavelength band in use, and a long-period grating provided in the multi-mode optical fiber, for selectively coupling fundamental LP01-mode light of a predetermined wavelength within the wavelength band in use to LP0m (mxe2x89xa72)-mode light. Specifically, the multi-mode optical fiber comprises a first core region of a refractive index n1 extending along a predetermined axis, a second core region of a refractive index n2 ( less than n1) disposed on a periphery of the first core region, and a cladding region of a refractive index n3 ( less than n2) disposed on a periphery of the second core region, and the long-period grating is provided in the first core region surrounded by the second core region. Such a multi-mode optical fiber may take such structure that an intermediate core region is provided between the first and second core regions or such structure that a depressed region is further provided between the second core region and the cladding region. In either of the structures,the multi-mode optical fiber applied to the optical fiber grating element has such structure that the cladding region is provided so that a propagation region A of a high-order mode is spaced away from the interface between the peripheral surface of the fiber and a coating material, as illustrated in FIG. 2.
Particularly, the optical fiber grating element according to the present invention is characterized by satisfying the relation of n1 greater than Neff01 greater than n2 greater than Neff0m greater than n3, where the effective refractive index with respect to the fundamental LP01-mode light is Neff01 and the effective refractive index with respect to the LP0m (mxe2x89xa72)-mode light Neff0m. With the optical fiber grating element of this structure, the fundamental LP01-mode light of the predetermined wavelength within the wavelength band in use is coupled to the higher LP0m (mxe2x89xa72)-mode light by the long-period grating formed in the first core region of the multi-mode optical fiber. Since the higher LP0m (mxe2x89xa72)-mode light is confined in the propagation region specified by the first and second core regions of the multi-mode optical fiber, it is little affected by the layer existing outside the multi-mode optical fiber (see FIG. 2). On the other hand, the fundamental LP01-mode light of the wavelengths other than the predetermined wavelength passes through the long-period grating provided in the first core region as it is. The fundamental LP01-mode light is confined in the first core region, whereas the higher LP0m (mxe2x89xa72)-mode light is confined in the propagation region specified by both the first and second core regions. Thus the mode field diameter of the multi-mode optical fiber is large for the higher LP0m (mxe2x89xa72)-mode light. In the case wherein a single-mode optical fiber is connected as a post stage of the optical fiber grating element, coupling loss is small when the fundamental LP01-mode light having passed through the long-period grating of the multi-mode optical fiber is incident to the core of the single-mode optical fiber. On the other hand, coupling loss is large when the higher LP0m (mxe2x89xa72)-mode light generated in the long-period grating of the multi-mode optical fiber is incident to the core region of the single-mode optical fiber.
With the multi-mode optical fiber of the above-stated structure, it is preferable to dope only the first core region with GeO2. The reason is that the coupling efficiency is increased in coupling from the fundamental LP01-mode light to the higher LP0m (mxe2x89xa72)-mode light. Since the second core region is not doped with GeO2, there is no change in the refractive index of the second core region before and after formation of the grating. For this reason, change is small in the mode field diameter of the multi-mode optical fiber with the long-period grating for the higher LP0m (mxe2x89xa72)-mode light, so that the optical fiber grating element can realize desired coupling characteristics and cutoff characteristics readily.
In the optical fiber grating element according to the present invention, the above multi-mode optical fiber may be coated at least around the peripheral surface surrounding the portion in which the long-period grating is formed. The reason is that since the higher LP0m (mxe2x89xa72)-mode light generated in the long-period grating is confined in the first and second core regions, variations are small in the wavelength at which there occurs the coupling from the fundamental LP01-mode light to the higher LP0m (mxe2x89xa72)-mode light, and in the coupling efficiency even if the grating-forming portion is coated. The coating is also effective in protecting the optical fiber grating element.
Particularly, if the layer covering the peripheral surface of the multi-mode optical fiber is an ultraviolet-transmissive resin, it eliminates the need for once stripping the coating layer as before. Thus the multi-mode optical fiber is prevented from externally being damaged, and the optical fiber grating element can be fabricated within short time.
Accordingly, a production method of the optical fiber grating according to the present invention is characterized by preparing a multi-mode optical fiber comprising at least a first core region of a refractive index n1 doped with a predetermined amount of GeO2, a second core region of a refractive index n2 ( less than n1), and a cladding region of a refractive index n3 ( less than n2) and covered with an ultraviolet-transmissive resin over the peripheral surface of the cladding region, as described above, and exposing this ultraviolet-transmissive resin to ultraviolet light, thereby forming periodic change of refractive index within the first core region. In this case, most of the ultraviolet rays impinging on the ultraviolet-transmissive resin pass the ultraviolet-transmissive resin, the cladding region, and the second core region in the stated order to reach the first core region.
The above wavelength band in use is preferably 1.2 xcexcm or more but 1.7 xcexcm or less. The wavelength band of this range enables use of the optical fiber grating element in the wavelength bands commonly used in optical communications. The multi-mode optical fiber is preferably one having normalized frequency of 4 or more but 12 or less at the above wavelength band in use. It is because this configuration enables the effective coupling from the fundamental LP01-mode light of the predetermined wavelength to the higher LP0m (mxe2x89xa72)-mode light and because it enables more precise design and fabrication of the optical fiber grating element with desired characteristics.
It is known that the long-period grating shifts its center wavelength of the loss band (loss peak wavelength) depending upon ambient temperature around the optical fiber grating element. In order to reduce such temperature dependence of loss peak wavelength, the optical fiber grating element according to the present invention is preferably one designed so that there exists at least one m satisfying the following relation:                     0.01        ⁢                  xe2x80x83                ⁢                  (                      μ            ⁢                          xe2x80x83                        ⁢                          m              /              xc2x0                        ⁢                          xe2x80x83                        ⁢                          C              .                                )                            Λ        ⁡                  (          μm          )                      ≥          "LeftBracketingBar"                                    ⅆ                          ⅆ              T                                ⁢                      (            Neff01            )                          -                              ⅆ                          ⅆ              T                                ⁢                      (            Neff0m            )                              "RightBracketingBar"        ,
where dNeff01/dT is a temperature dependence of the effective refractive index Neff01 with respect to the above fundamental LP01-mode light, dNeff0m/dT (mxe2x89xa72) a temperature dependence of the effective refractive index Neff0m with respect to the above LP0m (mxe2x89xa72)-mode light, and xcex9 a grating period of the above long-period grating. The above condition can be met, for example, by doping the first core region with at least either element of Ge, P, and B and properly adjusting a doping amount thereof.
It also becomes feasible to effect variable control of the loss peak wavelength (which will also be referred to hereinafter as temperature active control) by positively making use of such temperature dependence of the loss peak wavelength to the contrary. In this case, the optical fiber grating element according to the present invention is preferably one designed so that there exists at least one m satisfying the following relation:                     0.08        ⁢                  xe2x80x83                ⁢                  (                      μ            ⁢                          xe2x80x83                        ⁢                          m              /              xc2x0                        ⁢                          xe2x80x83                        ⁢                          C              .                                )                            Λ        ⁡                  (          μm          )                      ≥          "LeftBracketingBar"                                    ⅆ                          ⅆ              T                                ⁢                      (            Neff01            )                          -                              ⅆ                          ⅆ              T                                ⁢                      (            Neff0m            )                              "RightBracketingBar"        ,
where dNeff01/dT is a temperature dependence of the effective refractive index Neff01 with respect to the above fundamental LP01-mode light, dNeff0m/dT (mxe2x89xa72) a temperature dependence of the effective refractive index Neff0m with respect to the above LP0m (mxe2x89xa72)-mode light, and xcex9 a grating period of the above grating. The above condition can also be met, for example, by doping the first core region with at least either element of Ge, P, and B and properly adjusting a doping amount thereof.
An optical fiber filter according to the present invention comprises an optical fiber grating element having the structure described above, and a single-mode optical fiber having a cutoff wavelength regarding to LP02-mode light on the shorter wavelength side than the above wavelength band in use. This single-mode optical fiber is placed at least in a post stage of the optical fiber grating element when seen from a traveling direction of signal light of wavelengths included within the above wavelength band in use. In this optical fiber filter, the fundamental LP01-mode light of the predetermined wavelength within the wavelength band in use is coupled to the higher LP0m (mxe2x89xa72)-mode light by the long-period grating provided in the first core region of the optical fiber grating element. Since the higher LP0m (mxe2x89xa72)-mode light is confined in the propagation region specified by the first and second core regions of the multi-mode optical fiber, it is little affected by the external environment of the multi-mode optical fiber. In this optical fiber filter, the core-mode light of the predetermined wavelength is also coupled to the higher LP0m (mxe2x89xa72)-mode light, but the coupling loss is large when this higher LP0m (mxe2x89xa72)-mode light is incident to the core region of the single-mode optical fiber optically connected in the post stage. On the other hand, the fundamental LP01-mode light of the wavelengths other than the predetermined wavelength passes through the long-period grating of the optical fiber filter, but the coupling loss is small when this core-mode light is incident to the core region of the single-mode optical fiber optically connected in the post stage. Namely, the optical fiber filter functions to cut off (or selectively attenuate) the fundamental LP01-mode light of the predetermined wavelength within the wavelength band in use but transmit the fundamental LP01-mode light of the other wavelengths.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.